It is already known that in at least one operating range of the internal combustion engine, a deviation in the air-fuel mixture ratio from a setpoint value is corrected. Systematic errors in the air-fuel mixture composition are corrected at the same time by the mixture adaptation. Essentially a distinction is made between additive and multiplicative errors. These mixture deviations are adapted in the load speed range in which they have the greatest effect. They are then calculated into the entire load speed range. Additive mixture deviations which occur because of leakage air or fuel injector delay times, for example, are adapted in a lower load speed range. Multiplicative mixture deviations which occur due to a characteristic line drift of the air flow meter used, for example, are adapted in a middle to upper load speed range. A correction value is formed for each adaptation range, i.e., each load speed range in which an adaptation was performed, and this correction value is interpreted as a fuel error. In the case of an air error, e.g., due to a leakage in the intake manifold, this error is also corrected in the fuel path instead of in the air path.